1. Field of the Invention
This invention relates to manufacturing methods and vacuum or hermetically packaged micromachined or MEMS devices formed thereby having substantially vertical feedthroughs.
2. Background Art
Low-cost vacuum packaging of MEMS has become one of the most important challenges in the MEMS industry for a number of emerging applications including resonant sensors and RF MEMS. Many different packaging technologies have been developed over the years, including hermetic and vacuum packaging of MEMS. There are many requirements for a MEMS package. These include small size, vacuum/hermetic capability, integrated sealed feedthroughs, wafer level processing, compatibility with most MEMS processes, reliability, stability in the face of exposure to harsh environmental conditions such as temperature/humidity/vibration, and of course, above all, low cost. Unfortunately for the MEMS manufacturer, most of these requirements have to be satisfied at the same time. A package that cannot be fabricated at low cost, or is too large, or cannot provide feedthroughs, will not find many applications. This is where the challenge lies to further advance the field of MEMS sensors and Microsystems.
Many technologies have been developed for MEMS over the past few decades. One of these technologies is the so-called dissolved-wafer process (DWP) as described in U.S. Pat. Nos. 4,815,472 and 5,377,524. The process forms heavily boron-doped silicon microstructures on a glass substrate. Electrical connections are made to the silicon microstructures using metal lines deposited on the glass substrate. This process has been used to produce many different types of sensors, actuators, and Microsystems, and has been specifically used to manufacture one of the highest performance gyroscopes for inertial applications developed originally by Draper Lab. An important requirement for the successful operation of the gyroscope is that it should be operated in vacuum for many years. Therefore, a stable vacuum packaging technology is needed for this device.
S. Cho developed a vacuum package for this device based on bonding a glass capsule to the device glass substrate using an intermediate layer of high resistivity polysilicon as described in U.S. Pat. No. 5,837,562. Feedthroughs were formed from gold lines. During the bonding process, these gold lines interdiffused with the high resistivity polysilicon through a eutectic process to form sealed feedthroughs. The eutectic was able to reflow and therefore cover over the steps created by the gold lines. Although some data for the vacuum capability of this package was presented, no long term data has been presented. The electrical connections for this packaging approach were brought out as gold lines on the device glass substrate. Bonding pads were brought out to the edge of the chip and could be used for wire bonding. The fact that bonding pads were used on the same side as the device meant that additional die area was used. In addition, it was not easy to flip chip attach this die to a circuit board.
The following U.S. patents relate to semiconductor variable capacitance pressure transducers and methods of making same wherein one of the named inventors is also a co-inventor of at least one of the inventions described and claimed herein: U.S. Pat. Nos. 4,261,086; 4,277,814; and 4,386,453.